A hard disk drive (HDD) includes a disk on which data is stored, a spindle motor which rotates the disk, a read/write head, and a voice coil motor (VCM) to move the read/write head over the disk for reading/writing data from/to the disk surface. When the disk is rotating, and read/write operations are in progress, the head, which is carried on a VCM arm, is moved from a parked position to a position above the data storage surface of the disk. This is referred to in the art as “ramp loading” or “un-parking” of the VCM arm. When the disk is not rotating, the head must be parked in a safety zone away from the disk in order to prevent any damage to the disk or head. This is referred to in the art as “ramp unloading” or “parking” the VCM arm. When a power loss (any power supply inactivation such as a power failure event or a power-down event) occurs, it is important to move the read/write head to the safety zone away from the disk. If the VCM arm is not fully parked in the safety zone, the read/write head may land on the disk while the disk is spinning or stopped potentially resulting in damage to the disk and/or the head. It is thus critical to ensure full retraction of the VCM arm.
Those skilled in the art understand, however, that it is also important to precisely control the speed of the VCM when moving the VCM arm. For example, during retraction of the VCM arm a precise control over VCM speed is needed to ensure that the VCM arm carrying the read/write is not damaged if an obstacle is encountered. Conversely, during loading of the VCM arm and tracking of data on the disk, it is important to control VCM speed in order to ensure accuracy of read/write head placement over time.
The operation to park the VCM arm can present a challenge when a power loss (any power supply inactivation such as a power failure event or a power-down event) occurs. It is known in the art to harvest energy from the spindle motor in response to a power loss to power the operation of the VCM in retract mode. In such a case, the spindle motor is exploited as a generator whose output voltage is dependent on the speed of spindle motor rotation and the electrical constant of the motor itself. As the spindle motor slows, however, the amount of generated power begins to decrease. The generated power can be supplemented by energy stored in a capacitor during normal operation, but eventually the power available for use in VCM arm retraction will dissipate to zero, and the unloading of the VCM arm must be completed before that point is reached.
Because it is critical to complete the parking of the VCM arm while power remains available, this would suggest controlling the VCM for a very fast movement of the VCM arm to the parking zone. However, as discussed above, precise control over the speed of the VCM during VCM arm movement must be exercised so as to ensure that the retraction does not damage the head and/or arm. This would suggest controlling a slower movement of the VCM arm towards the parking zone. If the movement is too slow, however, the power available for retraction may fully dissipate before parking is completed, thus leaving the read/write head in a dangerous position over the disk.
There is accordingly a need in the art for a method and apparatus to control the speed of VCM operation during emergency retraction of the VCM arm so that the VCM arm retraction achieves parking on the ramp without moving too quickly. There is further a need in the art for adjusting the emergency retraction operation in response to the dwindling power availability.